1. Field of the Invention
The present invention relates to dynamic pressure bearing devices each equipped with a dynamic pressure bearing member that supports a rotation shaft with dynamic pressure generated by lubricating fluid, and the present invention also relates to motors equipped with such dynamic pressure bearing devices.
2. Related Background Art
In recent years, the development of dynamic pressure bearing devices in which rotation shafts are supported by having lubricating fluid generate dynamic pressure has been underway in order to create bearing devices that can rotate rotary bodies at high-speed and high precision in various types of rotary drive devices. In such dynamic pressure bearing devices, those having a thrust bearing section SB structured as indicated in FIGS. 8 and 9, for example, have been recently proposed in order to make the entire device thinner. With respect to the thrust bearing section SB indicated in these figures, a rotary member 3 is attached to a rotary shaft 2, which is supported in a freely rotatable manner by a dynamic pressure bearing member 1, and an inner end surface (bottom end surface) in the axial direction at the center part of the rotary member 3 is positioned opposite and in close proximity in the axial direction to an inner end surface (top end surface) of the dynamic pressure bearing member 1, thereby forming the thrust bearing section SB.
In the interior of the thrust dynamic pressure bearing section SB, an appropriate amount of a lubricating fluid (omitted from drawings) is filled and herringbone-shaped dynamic pressure generating grooves 4, for example, are formed concavely in a ring shape as a dynamic pressure generating means for the lubricating fluid, so that the pressurizing effect of the dynamic pressure generating grooves 4 causes dynamic pressure to be generated in the lubricating fluid and thereby yields a predetermined levitation force in the axial direction.
In order to effectively yield levitation force in the axial direction with the thrust dynamic pressure bearing section SB, it is desirable to set the pressurizing effect of the dynamic pressure generating grooves 4 inward in the radial direction (towards the center). Consequently, as indicated by arrows in FIG. 8, the pressurizing effect (i.e., the pumping effect) of groove sections 4a on the outer side in the radial direction is normally set larger than the pressurizing effect of groove sections 4b on the inner side in the radial direction, where the two sets of groove sections 4a and 4b comprise the dynamic pressure generating grooves 4, and the pressure difference between the two causes the lubricating fluid to be sent towards the center.
In the meantime, a fluid sealing section 5 comprising a capillary sealing section or the like is formed adjacent to an area on the outer side in the radial direction of the thrust dynamic pressure bearing section SB, in order to prevent the lubricating fluid within the thrust dynamic pressure bearing section SB from flowing out. The fluid sealing section 5 can be formed by making use of an outer circumference wall surface of the dynamic pressure bearing member 1, for example; more specifically, a tapered sealing space is created by forming an appropriate gap between the outer circumference wall surface of the dynamic pressure bearing member 1 and an inner circumference wall surface of a counter plate 6, which is attached to the rotary member 3 to serve also as a fall-out stopper member, and by gradually enlarging the gap towards an opening section at the bottom.
In forming the thrust dynamic pressure bearing section SB having such a structure, it is naturally desirable to reduce torque loss that occurs in the thrust dynamic pressure bearing section SB, and to that end efforts have been made to make the outer bearing diameter of the thrust dynamic pressure bearing section SB as small as possible. In other words, positioning the thrust dynamic pressure bearing section SB in the center part of an entire thrust opposing region that is formed between the end surface in the axial direction of the rotary member 3 and the end surface in the axial direction of the dynamic pressure bearing member 1 has been practiced for some time.
However, when the outer diameter of the thrust dynamic pressure bearing section SB is small, the pressurizing force (the pumping force) of the thrust dynamic pressure bearing section SB inward in the radial direction fails to work on the lubricating fluid that is in a region more outward in the radial direction than the thrust dynamic pressure bearing section SB; consequently, the lubricating fluid that is in the region more outward in a radial direction than the thrust dynamic pressure bearing section SB may scatter to the outside of the bearing (on the outer side in the radial direction) due to centrifugal force from rotation.
The rotational centrifugal force applied to the lubricating fluid is proportional to a square of rotational speed; consequently, when high-speed rotation of over 10,000 rpm takes place as in latest rotary drive devices, an extremely large centrifugal force is applied to the lubricating fluid, which makes it easy for the lubricating fluid to be scattered; the holding power of the fluid sealing section 5 may not be able to support this and there is a greater risk of the lubricating fluid leaking outside. When the lubricating fluid leaks outside, the amount of the lubricating fluid becomes insufficient inside the bearing by the amount leaked, which can shorten the life of the dynamic pressure bearing device. Further, in devices that require highly clean environment such as HDD (hard disk drive devices), the leaking of the lubricating fluid can cause contamination of internal equipment and lead to fatal problems for the entire device.
One way to prevent the lubricating fluid from leaking outside of the thrust dynamic pressure bearing section SB is to make gaps between members positioned near the thrust dynamic pressure bearing section SB narrow, but this causes rotational torque loss in narrowed pathway sections, which can lead to such problems as increased drive current.